The invention is directed to a light source element having a light wave guide for back-lighting of liquid crystal displays and for ambient illumination.
One important job in the back-lighting of liquid crystal displays is comprised in illuminating the liquid crystal display area with an optimally uniform, monochromatic or polychromatic light radiation having an adequately high luminance. For this purpose, the light radiation emitted from one or more light sources must, on the one hand, be as uniformly distributed onto the display area as possible, whereby, on the other hand, the losses should be optimally minimized.
EP-A-0 500 960 discloses a planar light source element that is to be utilized for back-lighting in a liquid crystal display. Given this light source element, a light source is arranged at an end face surface as a light entry face of a transparent light waveguide. A surface of the light waveguide that is perpendicular to the light entry face serves as a light exit face, and a light-reflecting layer is arranged at that surface of the light waveguide lying opposite this light exit face. Further, a diffusing element is arranged such that the light emerging from the light exit face is diffusely scattered. The homogenization of the light radiation over the surface of the light source element is then achieved in that one or both surfaces of the light waveguide comprises roughened and plane sections, and the area ratio of the roughed to the plane sections is continuously varied along the waveguide. The plane sections have the property that, due to total reflection, light rays are reflected back into the waveguide, whereas the light rays are scattered at the roughened sections. Since the luminance is initially relatively high at the light entry side of the light waveguide, a relatively high proportion of plane surfaces is set thereat, so that there is a relatively high probability that the light waves in this region will propagate by multiple total reflections in the waveguide. This areal proportion of plane sections is continuously reduced over the course of the waveguide, so that the light radiation can be increasingly scattered at the increasing proportion of roughened surfaces. Generating a relatively uniform output radiation at the light exit face of the light source element is thereby achieved.
Given the described arrangement, the light radiation must be coupled into the light waveguide at an end face of the light source element. Given employment of a fluorescent tube arranged along this side and surrounded by a metallic reflector, an adequate luminance for the back-lighting of a liquid crystal display can presumably be offered in many cases. Nonetheless, this arrangement is relatively inflexible since the luminance cannot be increased above a certain degree due to the limitation in view of the employable light source. Moreover, the attachment of the light source to the lateral end face of the light source element is also unfavorable for space reasons because the space required therefor ultimately limits the width of the display area of the liquid crystal display.
The described arrangement is suitable, for example, for the back-lighting of a liquid crystal display at the dashboard of a motor vehicle. When the display is located relatively far up on the dashboard, the display area is highly visible. When, however, this is arranged relatively far down on the dashboard, the driver or passenger views the display area with a relatively large observation angle. This leads to faulty recognition of the display area, particularly given liquid crystal displays with their great dependency on observation angle.
Given the described arrangement, a foil or film with a vapor-deposited metallic layer is applied as a light-reflecting layer onto the surface of the light waveguide lying opposite the light exit face after the manufacture of the light waveguide. This procedure of applying the film, however, proves to be relatively involved, since the film must generally be glued onto the surface of the light waveguide. An adhesive must be employed for this purpose that should optimally exhibit adequate transparency for a broad wavelength range of the visible spectral range since the light source element should be employable not only for the back-lighting of liquid crystal displays with white light sources but also for monochromatic back-lighting with LEDs having an arbitrary wavelength.
The arrangement disclosed by EP-A-0 500 960 is therefore suitable for a good lighting situation only when certain preconditions are met.